Polybenzazole represented by poly(paraphenylene benzobisoxazole), poly(paraphenylene benzobisthiazole), poly(paraphenylene benzobisimidazole) and the like is a polymer represented by the following formula (c), and known to be obtained by condensation polymerization of compounds (a) and (b) as shown in the following reaction formula (IV). ##STR1## wherein each Ar is an aromatic group, each Y is a functional group having electron-deficient carbon, such as carboxyl, haloacyl, haloalkyl and nitrile, and each Z is --O--, --S-- or --NH--.
The compounds (a) and (b), which are the starting materials of polybenzazole, are generally called AA-PBZ monomer and BB-PBZ monomer, respectively.
U.S. Pat. No. 5,276,128 discloses, besides a production method of polybenzazole from the above-mentioned AA-PBZ monomer and BB-PBZ monomer as starting materials, a production method using, as a starting material, a salt of PBZ monomer of the formula ##STR2## wherein each Ar is an aromatic group and each Z is --O-- or --S--. This PBZ monomer salt gives polybenzazole through condensation polymerization thereof.
The polybenzazole thus obtained is utilized to give a fiber, a film and the like. The properties thereof such as strength and elastic modulus vary according to the polymerization degree of the polymers. For a stable production of the polymer, the polymerization degree should be accurately controlled during the production process. Accordingly, control of the polymerization degree of the polymer to be obtained ultimately and of the polymer during the production process is one of the important techniques for the production of polybenzazole.
In the production of polybenzazole using a PBZ monomer salt of the above-mentioned formula, since the composition molar ratio of the AA-PBZ monomer and BB-PBZ monomer is 1:1, 100% completion of the reaction theoretically makes the polymerization degree of the polymer finally obtained infinite. For a desired polymerization degree to be achieved, the degree of reaction should be controlled. While the degree of reaction can be controlled to some extent by changing reaction conditions such as polymerization temperature and reaction time, this route is highly likely associated with the risk that the production may not proceed due to an excessive increase in the polymerization degree at the residence part within a reactor or a transfer tube, as well as difficulty in responding to various production amounts. Consequently, it is practically very difficult to accurately control the polymerization degree by way of reaction conditions alone.
The above-mentioned U.S. Pat. No. 5,276,128 also discloses a method for controlling polymerization degree by the use of a chain-end terminator. In this case, the polymerization degree of the final polymer can be controlled, whereas control of the polymerization degree of the polymer during production process is still difficult to achieve, and unless the reaction degree of the polymer at the time when the end terminator is added is accurately controlled, the method is remotely practical in view of inconsistent molecular weight distribution of the obtained polymers.
Thus, there has not been known an effective method for controlling polymerization degree in the production of polybenzazole from a PBZ monomer salt as a starting material.
It is therefore an object of the present invention to provide a method for accurately controlling the polymerization degree in the production of polybenzazole from a PBZ monomer salt as a starting material, wherein the composition molar ratio of the AA-PBZ monomer and BB-PBZ monomer is 1:1.